OPTIMUM DESIGNS FOR FISH BREEDING PROGRAMS WITH CONSTRAINED INBREEDING - MASS SELECTION FOR A NORMALLY DISTRIBUTED TRAIT

Optimum designs for fish breeding programmes to achieve maximum geneti c gain whilst constraining the rate of inbreeding are studied through stochastic simulation. Mass selection is carried out for 15 discrete g enerations for a trait recorded in both sexes. Schemes are optimized f or the number of males selected. Different but fixed numbers of scored individuals, heritabilities and mating ratios are analyzed. Genetic g ains averaged over the last ten years of selection obtained when impos ing specific constraints on the rate of inbreeding are compared. Varia bility of response is also considered. For a fixed mating ratio, the o ptimum number of sires increases with the size of the scheme, the heri tability and with more severe restrictions on the rate of inbreeding. The effects of the size of the scheme and the heritability on the opti mum number of selected individuals are larger when more severe constra ints on inbreeding are imposed. Increasing the mating ratio leads to l ower genetic gains and to a considerable higher number of fullsib grou ps. However, the effect of mating ratio on genetic gain is very small. The increase in genetic progress obtained by increasing the size of t he scheme is higher with more severe restrictions on inbreeding and wi th higher mating ratios. In percentage, the loss in gain by restrictin g inbreeding to different levels is very similar for different heritab ilities and is larger for the smaller schemes. The reduction in geneti c progress by decreasing the rate of inbreeding from 2% to 0.25% range d from around 10% (largest scheme) to around 40% (smallest schemes). T he extra genetic progress obtained by increasing the size of the schem e was larger for the smaller schemes, the higher heritabilities, the h igher mating ratios and under the more severe restrictions on inbreedi ng. The variance of genetic gain decreased with more severe restrictio ns on inbreeding and with the heritability and remained approximately constant for schemes of different size and for different mating ratios . The implications of the results for fish breeding programmes that ap ply mass selection are discussed.